Air-to-refrigerant heat exchangers are commonly used in air conditioning and refrigeration systems to exchange heat between a refrigerant and air as the two fluids flow through the heat exchanger. In general, the higher the air flow rate through the heat exchanger, the better the heat transfer performance of the heat exchanger. The typical air-to-refrigerant heat exchanger used in an air conditioning or refrigeration system is of the fin and tube type. In a fin and tube heat exchanger, refrigerant flows through a closed flow path within an arrangement of tubes in the heat exchanger. Air flows over the exterior of the tubes. There are a plurality of fins extending from the exterior surface of the tubes in order to increase the surface area and thus the heat transfer performance of the tube. Other variables being equal, there must be a certain minimum air flow through a heat exchanger having a given refrigerant-to-air heat transfer area for the system that the exchanger serves to be capable of performing to its rated capacity.
Designers of air conditioning systems are constantly engaged in efforts to improve their products. One common design objective is to provide the maximum possible cooling or heating capacity in the smallest possible enclosure or the space available. Almost inevitably, configuration changes that improve one feature of a system lead to problems in another. For example, a heat exchanger designer may find it desirable to reduce the overall volume and face surface area of a heat exchanger, while maintaining the heat transfer area necessary to attain required capacity, by arranging the tubes of the heat exchanger in multiple rows. As the number of tube rows increases, the resistance to air flow through the heat exchanger also increases. Thus, increasing the number of tube rows through which the air in a heat exchanger must pass makes the task of the designer of the air movement portion of the system more difficult as that designer must provide a fan arrangement that can provide the necessary air flow rate through the heat exchanger. Resistance to air flow may also be caused by changes in the fluid path that the air flow must take.
To overcome the pressure loss through a multi-tube row heat exchanger, the fan that moves air through the heat exchanger must produce a relatively high differential pressure in the air flowing through it. Pure axial flow fans are not generally capable of producing the required differential pressure without severe compromises in performance. For instance, if an axial flow fan having a relatively small hub and long blades is used in such an application, there will be large losses at the periphery of the swept area of the fan impeller. These losses can be avoided by using an axial flow fan with a relatively large hub and short blades, but then the distribution of air flow across the heat exchanger will be less than optimal and the system thermal performance will suffer. Some of the losses associated with producing high differential pressures with an axial flow fan can be reduced by making the clearance between the tips of the fan impeller and the surrounding orifice defining shroud very small. Achieving the necessary small clearance in a typical manufacturing and assembly operation can be difficult and expensive and the designer must take steps to insure that the clearance can be maintained throughout the life of the system with little or no maintenance.
A mixed flow fan combines in a single fan the flow characteristics of both axial and centrifugal flow fans. In such a fan, a portion of a given impeller blade imparts axial movement to the air flowing through the impeller while another portion of the blade imparts centrifugal movement. Such a fan is capable of creating relatively high differential pressures when operating with a relatively high downstream flow resistance and therefore relatively high air flow rates when compared to, for example, a solely axial flow fan operating in a similar environment. Prior art mixed flow fans have typically had impeller hub shapes that promote a transition in the air entering and flowing through the fan from an axial to a radial direction. These hub shapes generally increase in diameter in an upstream to downstream direction. Such hubs present manufacturing problems, especially if a fan impeller is to be made of plastic by a molding process. The performance of a mixed flow fan is less sensitive to impeller blade tip to shroud clearance than an axial flow fan.
What is needed is a fan in combination with a heat exchanger having a relatively high air flow resistance where the fan can efficiently produce the required air flow through the heat exchanger. The configuration of the fan impeller should be such that the impeller can be made by a molding process.